The conventional pushbutton switch of the alternate type, commonly called a "push switch", as described as prior art in U.S. Pat. No. 4,740,661 issued to the same assignee as the present application, was troublesome in assembling because of its complicated construction wherein an actuator pin is actuated by a pushbutton to pivotally drive a switching member having a pushrod outwardly biased by a spring, which pushrod in turn pivotally drives a movable contact support carrying a movable contact piece to thereby selectively bring a common contact connected to the movable contact piece into electric contact with either one of two fixed contacts.
In addition, the pushbutton switch had the disadvantage of low reliability in operation because of contact failure tending to occur due to dust sticking to the contacts through a long period of time as the movable contact piece and fixed contacts are brought into contact with each other always at the same portions.
In an attempt to solve the foregoing problems, the present applicant proposed an improved pushbutton switch of the alternate type in Japanese Utility Model Application No. 32642/1992 (4-32642) filed May 19, 1992. This earlier proposed pushbutton switch is illustrated in FIG. 1 as a vertical cross-sectional view and in FIG. 2 as a ninety degree turned cross-sectional view taken on line II--II of FIG. 1.
This switch comprises an insulating case 13 having a bottom opening closed by a bottom panel 14 through which contacts 15, 16 and 17 extend out as terminals. Mounted on the bottom panel 14 is a movable contact support 19 for seesaw motions with the lower end of a projection extending from the bottom surface of the support 19 at the center thereof as a fulcrum. Movable contact pieces 18 bent in the shape of an inverted U are retained in corresponding accommodating grooves 19S formed in the bottom surface of the support 19. The seesawing movement of the movable contact support 19 causes the movable contact pieces 18 in unison with the support 19 to pivot in resilient contact with the common contacts 15 extending inwardly from the bottom panel 14 whereby the opposite ends of the movable contact pieces 18 are slidingly moved into and out of the corresponding fixed contacts 16 and 17.
The movable contact support 19 has a mountain-like transverse ridge or crest raised from its top surface with an apex lying in the center of the surface such that the oppositely downwardly sloping surfaces 19A and 19B of the ridge define slide guide paths for the tip of a pin 21. Extending from the lowermost ends of the respective inclined surfaces 19A and 19B toward the opposite ends of the support 19 are upwardly sloping surfaces 19AA and 19BB which define abutment surfaces. The movable contact support 19 has transverse recesses 19R formed in the opposed side walls 19W (see FIG. 2) centrally of the length between which the inclined surfaces 19A and 19B transversely extend.
Resting on the movable contact support 19 is a spring retainer plate 22 having opposed spaced apart triangular plate-like supporting projections 22T protruding from the undersurface thereof, the projections 22T in being engagement with the corresponding recesses 19R of movable contact support 19 to permit the seesaw motion of the spring retainer plate 22.
The pin 21 passes through a slot 22A formed through the spring retainer plate 22 and has a flange 21A formed at an upper end thereof. The flange 21A is engaged by the upper end of a frusto-conical coil spring 23 and resiliently urged thereby against the bottom wall (the most recessed portion) of a pin receiving hole 12A formed in the lower end surface of a pushbutton 12 which is in turn mounted in a pushbutton housing section 11 of the insulating case 13. The lower end of the coil spring 23 is securedly supported by the top surface of the spring retainer plate 22.
With the pushbutton 12 in its outermost position, the lower end of the pin 21 is in spaced and opposed relation to either one of the inclined surfaces 19A and 19B, the surface 19B, for instance. As the pushbutton 12 is depressed, the lower end of the pin 21 is brought into contact with and slides down the one inclined surface 19B until it abuts against the abutment surface 19BB to turn the movable contact support 19 into the oppositely tilted orientation while at the same time inverting the tilted orientation of the spring retainer plate 22, with the result that the contacts of the movable contact pieces 18 with the fixed contacts 16 and 15 are inverted.
Upon the pushbutton 12 being released, the pushbutton 12 and pin 21 are restored by the coil spring 23 into their initial positions while the tilted orientation of the movable contact support 19 is maintained as it is by the spring retainer plate 22, so that the lower end of the pin 21 is in spaced and opposed relation to the other inclined surfaces 19A.
In the illustrated example, since the spring retainer plate 22 is constructed so as to be freely swingable, it not only supports the lower end of the spring 23 on the top surface thereof but also acts as a switching member which can change over the direction of the seesaw motion of the movable contact support 19.
With this construction, each time the pushbutton 12 is pushed down, the pin 21 is brought into direct contact alternately with the inclined surfaces 19A and 19B of the movable contact support 19 which guide the tip end of the pin 21 toward either of the abutment surfaces 19AA or 19BB of the movable contact support 19 adjacent the opposite ends thereof, whereby downward force is applied alternately to the ends of the movable contact support 19 to seesaw the movable contact support 19 and hence the movable contact pieces 18. The seesaw motion of the movable contact pieces 18 brings the common contacts 15 into electric connection alternately with the fixed contacts 16 and 15, thus effecting the alternate operation.
The earlier proposed pushbutton switch as described above is characterized in that the arrangement in which the spring retainer plate 22 is acted on by the spring 23 so as to hold the movable contact support 19 in its stable state in either one of the two tilted positions is independent of the arrangement in which the movable contact support 19 is actuated by the lower end of the pin 21.
It is to be noted that the spring retainer plate 22 is arranged to be freely swingable and that the biasing force of the resilient spring 23 is transmitted to the movable contact support 19 by means of the freely swingable spring retainer plate 22 to impart the downward force (force directed toward the bottom panel 14) to the seesawing movable contact support 19. This downwardly directed force ensures the "bistable" seesawing motion of the movable contact piece 18.
In addition, the construction of the movable contact piece 18 described in the Japanese Utility Model Application No. 32642/1992 ensures that the movable contact piece 18 slidingly moves relative to the fixed contacts 16, 17 to effect what is called "self-cleaning action" as it is moved into and out of contact with the fixed contacts. This movable contact piece thus provides the advantage that it prevents contact failure from occurring for a long time of use, thereby enhancing the reliability of the contacts.
In assembling the earlier proposed pushbutton switch, the pushbutton 12, pin 21, coil spring 23 and spring retainer plate or switching member 22 are successively (in the order named) inserted into the insulating case 13 with the bottom opening thereof facing upward, then the movable contact support 19 with the movable contact pieces 18 supported therein is inserted into the case, and finally the bottom opening is closed by the bottom panel 14 to complete the assembly.
It has been found, however, that the spring retainer plate 22 cannot sometimes be assembled in proper position relative to the coil spring 23, since the spring retainer plate 22 is not positively anchored in position so as to be free to swing in the insulating case 13. For this reason, the time required for the assembly tends to be undesirably prolonged.